I shot a dead riser at a bumper behind me, trying to rebound the shot. Intead, it passed through the bumper. I'm not lying, neither was I using a bot(when a tank quits). This has happened before too, and I've observed it happens when you shoot quickly.
It possible if you caught the very edge of the bumper it didnt register.... Ive had that happen (because of a bad angle)
I have seen a lot of weapons go through the bumpers. Usually it's a spread shot like fireworks or pyrotechnics. It's usually just a few things but not everything though.. it's like it reflects some and lets some just go through. I'll try to screen shot it next time I see it.